This invention relates to sample concentration for gas chromatography and specifically to inhibition of foam in samples that are concentrated and analyzed using gas chromatography techniques.
A purge and trap sample concentrator is a device that may be used to extract volatile and semi-volatile analytes from a sample by passing a purge gas through the sample and trapping analytes onto a sorbent bed or trap. Subsequent desorption from the trap, and transfer of the desorbed analytes to a gas chromatograph, allows for separation and quantitative analysis of the volatile analytes.
One problem in the extraction process is that some samples have a tendency to form a foam which, if unabated, can be transferred throughout the purge and trap system, and may contaminate the entire system and require the entire system to be serviced, cleaned, revalidated, and/or calibrated.
One approach to this problem is pre-treating suspect samples with de-foaming solutions such as polypropylene glycol 2000, or Dow Corning Silicone RID emulsion.
However, this pre-treating can alter the extraction process itself, raising concerns regarding sample integrity, particularly for analytes that are highly volatile.
A purge and trap sample concentrator includes a sparge vessel in which an aliquot of a sample is placed. A first type of sparge vessel has a generally U-shaped tube, with a frit installed in a first leg or side of the U-shaped tube. A sample may be introduced into the first leg or side of the sparge vessel having the frit. Couplings may be attached to each side of the sparge vessel to create a sealed system. The output coupling is attached to the leg or side of the sparge vessel having a frit. The output coupling may contain a valve and needle assembly, a port, and additional components such as temperature sensors. The valve and needle assembly are for sample introduction and removal (draining). The port is to direct the purged analytes from the sample through tubing to the sorbent bed or trap.
A second type of sparge vessel, often referred to as a needle sparge assembly, has a sealed tube with a valve and needle that serves as a sample inlet, sparge flow inlet, and drain tube. Sparge gas is passed through the needle to generate bubbles and sparge the sample of the volatile and semi-volatile analytes.
Purging analytes from either type of sparge vessel may result in foam. The action of the frit in the first type of sparge vessel is to generate small gas bubbles across the top of the frit. In the needle sparge assembly, the gas that elutes from the needle generates bubbles. The small bubbles enhance the extraction efficiency of the gas passing through the sample. However, a potential side effect of this aeration process is the formation of foam. Generally, bubbles rupture at the liquid interface at the top of the sample. However, some samples retain the bubbles as an aggregate (foam) and this aggregate may be transported throughout the sample concentrator system. This may contaminate the system and necessitate cleaning.
Various designs of sparge vessels have been made to reduce the propensity for foam propagation. For example, for large volume bubbles, expanding the diameter of the sparge vessel (above the air-liquid interface) may help the bubbles rupture due to expansion forces becoming greater than the surface tension of the bubble. For highly viscous samples, a mud-dawger, which is a pointed element on a needle or other structural element in the sparge vessel, may be used to rupture the bubbles. However, these solutions are especially ineffective for small bubbles that flow around or through the potential mechanical foam disruption elements.
Other attempts have been made to detect foam in a sparge vessel. For example, WO 01/69264 discloses a conductivity sensor within a sparge vessel for the detection of foam. The conductivity sensor provides an alarm when foam is detected, permitting the user to dispose of the sample without continued analysis. The device in WO 01/69264 may detect foam without eliminating or reducing the foam. Also, the conductivity sensor may not detect foam that is not conductive.
Thus, a device and method are needed to detect, reduce and/or eliminate foam that is in sparge vessels in purge and trap sample concentrators.